The increased use and reliance on microprocessor control systems for automotive vehicles and increased confidence in hydraulic as opposed to mechanical systems is making substantial progress in engine systems design possible. One such electrohydraulic system is a control for engine intake and exhaust valves. The enhancement of engine performance to be attained by being able to vary the timing, duration, lift and other parameters of the intake and exhaust valves' motion in an engine is known in the art. This allows one to account for various engine operating conditions through independent control of the engine valves in order to optimize engine performance.
For proper starting of an internal combustion engine, suitable motion of the engine valves should begin as soon as the crankshaft begins its rotation. While electrohydraulic engine valve control systems in vehicles provide more flexibility to increase engine performance, they require a constant supply of pressurized hydraulic fluid to operate. Since the hydraulic engine valve control systems need to be operable when the engine is being started, even after the vehicle has been left idle for sometime, this requires a supply of pressurized fluid during engine start-up.
In a conventional camshaft driven valvetrain, proper motion of the engine valves is assured by mechanical connection between the crankshaft and the camshaft. In an engine with an electrohydraulic valvetrain, the mechanical link between the crankshaft and the engine valves is partially or completely absent and the proper motion of the engine valves can take place only when there is adequate hydraulic pressure in the system. If there is not adequate pressure in the hydraulic system, then proper engine valve motion cannot occur until the pressure is increased. Thus, a lack of pressure will delay engine start-up.
To assure fast start of an engine equipped with an electrohydraulic valvetrain, the system should remain filled with pressurized fluid even when the engine is not running. This pressure will prevent formation of fluid vapor bubbles and will cause the fluid to retain a relatively high value of the bulk modulus. Such a system, completely filled with hydraulic fluid, would require very little additional oil to increase its pressure. By maintaining system pressure, as soon as a hydraulic pump affixed to the hydraulic system begins to operate during engine start-up, there would be sufficient pressure to operate the engine valves, and within one or two pump revolutions the system could achieve its full operational pressure required for full engine operation. On the other hand, a hydraulic system that is not completely filled with pressurized fluid due to leakage during non-operating periods or due to contraction of the fluid from cooling off during non-operating periods, risks the formation of vapor bubbles and consequently cannot assure the immediate operation of the engine valves during engine start-up.
The need arises, then, in an engine with an electrohydraulically controlled valvetrain, to assure that there is hydraulic fluid pressure in the engine valve hydraulic system that allows immediate engine valve operation during start-up to ensure a fast start of the engine. This hydraulic system should maintain the pressure even if the engine has been idle for a period of time.